1. Field of the Invention
The present invention relates to a PLL (phase locked loop) circuit for producing a clock signal and more particularly to a PLL circuit for producing a clock signal which is synchronized in phase with a clock component contained in the read signal from a disk and for deriving it as a reproduction clock signal
2. Description of Background Information
As a method of recording information to a disk-shaped recording medium, a CAV (constant angular velocity) method and a CLV (constant linear velocity) method are known The disk based on the CLV recording method (hereinafter, simply referred to as a CLV disk) has an advantage such that it is possible to record information of an amount which is about twice as large as that in the case of the disk based on the CAV recording method. Therefore, the CLV recording method is used in a digital audio disc called a CD (compact disc) and a CD-ROM whose fundamental recording format is the same as that of the CD and which is used as a recording medium of digital data
To read the recorded information from the CLV disk, it is necessary to produce clocks which are synchronized in phase with the clock component included in the read signal from the disk. A PLL circuit is widely used to produce such clocks In recent years, to promptly read out desired data from the CD-ROM, the development of the high-speed access of the CD-ROM is in progress. However, even if the accessing speed is made higher, in handling data of a large amount such as data of image information or the like, it takes a relatively long time to read the data, and a limitation is imposed in reducing the total time which is required to read out the data. Therefore, it is necessary to raise the speed of reading the recorded information from the CD-ROM. To raise the reading speed of the recorded information from the CD-ROM, the recorded information needs to be read out at a higher speed, for instance, at a linear velocity faster than the ordinary linear velocity by two times, four times, or the like.
As mentioned above, in reading the recorded information at a linear velocity which is twice, four times, or the like faster than the ordinary linear velocity in order to realize the high speed reading of the data from the CD-ROM, an oscillating frequency of a VCO (voltage controlled oscillator) in the PLL circuit for producing clocks also should be changed in accordance with the linear velocity For this purpose, a method is conceivable wherein a plurality of VCOs of the oscillating frequencies corresponding to the linear velocities are provided and switched over in accordance with the linear velocity. However, with such method the circuit construction becomes complicated and also the cost becomes high.